Reptiles are terrestrial vertebrates (the majority, there are also some aquatic) tetrapods that evolved from amphibians about 270 my ago In the Mesozoic Era they dominated all environments, but many became extinct, like the dinosaurs, when the Era ended.
They are tetrapods, so most reptiles, such as lizards and tortoises, have four legs. These legs are usually short and end in five toes with nails. Other reptiles, such as snakes, are limbless, although because of their evolutionary parentage they continue to be classified as tetrapods.
Reptiles are ectotherms, so they cannot regulate their temperature. They tend to live in warm places and are lethargic when it is cold.
When reptiles grow, they have to molt, shedding old skin. Snakes often lose all of their skin.
Evolution has led them to have a series of adaptations that have allowed them to become independent of water.
They have pulmonary respiration. Their lungs are more complex than those of amphibians, so they take in more oxygen and do not need skin respiration. That is why they have been able to cover their body with horny scales that waterproof them against desiccation. In some cases, such as in turtles, the scales join together to form a very hard shell.
The sense organs are grouped mainly in the head. Most of the receptors for taste, smell, and touch are concentrated on the tongue.
The reptilian heart consists of two atria and one ventricle, which is partially septate in more evolved reptiles.
Most reptiles are carnivorous, killing their prey with their teeth. Some kill their prey by suffocation or by injecting it with poison before swallowing it. Some turtles, lizards, and iguanas are herbivores.
Most are oviparous. Their eggs have amnion and a shell that prevents desiccation during embryonic development, which takes place on land. Others, like the viper, are ovoviviparous, and the eggs hatch inside the body of the female.